Field of the Invention
The present invention relates to an image capturing system that applies assist light to an image sensing field to detect focus, an illumination apparatus of the image capturing system, and a method for detecting focus in the image capturing system.
Description of the Related Art
There is a known technique in the related art for autofocusing (AF) in an image capturing apparatus while applying assist light (hereinafter referred to as AF assist light). Examples of a method for applying AF assist light include applying patterned LED light from an LED light source and applying flash light from a discharge tube of an illumination apparatus.
Japanese Patent Laid-Open No. 2002-139664 discloses a technique for controlling driving of a lens using the result of focus detection having higher reliability of the result of focus detection using flash assist light and the result of focus detection using LED assist light.
Although LED assist light is effective for low-contrast subjects because it is applied in a pattern, its coverage is limited as compared with flash assist light. In contrast, flash assist light has wide coverage and is therefore effective for a wide range in a photographing frame but is ineffective for low-contrast subjects. Thus, the technique disclosed in Japanese Patent Laid-Open No. 2002-139664 aims at enhancing AF accuracy in consideration of the advantages and disadvantages of the methods of radiation.
In photographing using an illumination apparatus, such as a flash unit, an optical filter and so on (hereinafter referred to as optical accessories) are sometimes disposed in front of a light emitting unit of the flash unit. Examples of the optical accessories include a color filter for adjusting the color temperature of radiated light to match the color of the subject to the color of the background (changing the chromatic characteristics of the light) during photographing under the light of an incandescent lamp (tungsten), a diffuser for scattering radiated light to attain soft lighting, with the light emitting unit oriented to the subject, and a wide panel that increases light distribution to cope with wide angle lenses.
The technique disclosed in Japanese Patent Laid-Open No. 2002-139664 does not allow for control of AF assist light when such optical accessories are used.